An animal acting as a male first uses a syringe-like organ to stab its partner and inject prostate fluid into its body. The “male” then inserts its penis into the partner’s genital opening; the penis has spines that anchor it in place, but harm the other slug.

Because mating is so traumatic for the “female”, the slugs prefer to act male, and often resist mating altogether. But curiously, they still mate as females much more often than is necessary simply to ensure that their eggs are fertilised, says Rolanda Lange of the University of Tübingen in Germany.

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Lange and colleagues captured groups of sea slugs and gave different groups more or fewer opportunities to mate. The slugs produced the most eggs when they acted as females at a medium rate.

In theory, slugs should act as females just often enough to maintain a store of sperm, and no more. But the slugs mated as females much more often than that. Yet they produced the same proportion of fertilised eggs regardless of how many mating opportunities they had. This indicates that even the slugs that mated the least had gathered more than enough sperm for their reproductive needs.

All of this suggests the traumatic mating has some benefit that goes beyond reproduction – an advantage that offsets the bodily harm. We don’t know what that might be, says Mike Siva-Jothy of the University of Sheffield, UK. But the injections of prostate fluid might include nutrients that benefit the stabbed slug.

Male insects and spiders often proffer food as a bribe to persuade females to mate. “Male” sea slugs might be doing the same thing. “Males are giving with one hand and taking with the other,” Siva-Jothy says.